Wood is a natural, sustainable material. Wood has long been used as a building material by human beings. Different from solid wood lumber, wood composites use small wood units such as veneer, chips, fibers, flakes and particles that can be used in the manufacture of large panel materials to meet customer's requirements. They are also flexible in dimensions compared to solid lumber, which is limited by the diameter and shape of wood logs. Due to the development and use of synthetic wood adhesives in the early 1940s, the durability of wood composites has significantly improved.
Since wood is a hygroscopic material, it absorbs water or moisture through its vessel elements, fiber tracheids, and other cavities existing in the microstructure. In addition, wood can swell and shrink due to the movement of water or moisture in its cells and cavities by the capillary action in a natural atmosphere. These defects are harmful to strength properties and durability of wood and wood composites. For example, an oriented strand board (OSB) bonded with a Type I adhesive such as phenol formaldehyde (PF) has an improved water resistance, but it is easily swollen and even delaminated into layers across the board thickness within a certain period of outdoor exposure. Furthermore, wood is subject to the attack of fungi and other biodeterioration agents in wet conditions.
For better outdoor services, wood and wood composites are normally covered with a coating material. The most common coatings for wood and wood composites include latex, acrylic, alkyd, and urethane coatings. Latex is usually a vinyl acetate (VAc)-based paint, in which acrylic can be added as a copolymer to improve the wearing and sunlight resistances of latex. VAc is a very good binder to most substrates, easily mixed with various additives (e.g., pigments) and is lower in cost. However, it is poor in water resistance and not very durable. Acrylic coating uses acrylic polymers as the primary component. It is good in water and wearing resistances. Latex and acrylic both are normally waterborne paints. Acrylic polymers can also be emulsified as emulsion paint. Alkyd and urethane coatings are normally oil-based paints and made from polyester and polyurethane, respectively. Among these coatings, latex is lower in cost than acrylic, alkyd, and urethane coatings.
Both latex and acrylic coatings can be quickly dried out within a couple of hours at room conditions, while the alkyd and urethane paintings usually take one or more days to be completely cured. In addition, the alkyd painting is brittle and may turn yellow later if the coated wood is stored in a dark place. Although the above coatings have a good bond to wood, they need to be repainted every 2 to 4 years to repair cracking, chipping, and peeling during outdoor exposure.
Polyvinyl acetate (PVAc) is a thermoplastic polymer, which is made from free radical polymerization of the monomer vinyl acetate (VAc) under an initiator. PVAc is normally available as an emulsion adhesive that includes polymerized vinyl acetate. The emulsion may also include some fraction of monomers such as VAc and butyl acetate (BAc) and a copolymer such as polyvinyl alcohol (PVA) and ethylene vinyl acetate (EVA) (EVA is mostly used as a copolymer of PVAc for coating, book binding, and other applications). These ingredients are further blended with water, a biocide or preservative, and the like. In order to maintain a good emulsion stability of polyvinyl acetate, a nonionic or anionic surfactant may also be added.
For crosslinking polyvinyl acetate (XPVAc, or x-PVAc), a crosslinking agent or crosslinker (also called an adhesive modifier) is usually added into the resin emulsion. The crosslinking mechanism of XPVAc at the wood-adhesive interface is not yet fully understood. By the initiation of an acidic catalyst, the crosslinking agent may react with PVAc, PVA and lingocellulose to form a crosslinking structure at the interface. Additionally, the free hydroxyl groups (—OH) of PVA and lignocellulose and acetate groups (—OOCCH3) of PVAc may form hydrogen bonds together. Moreover, there may exist a mechanical interlocking among the crosslinked/uncrosslinked molecular chains of PVAc and PVA, thus resulting in a complex network structure at the interface which significantly improves the water resistance of XPVAc.
XPVAc has been used as a wood adhesive in the wood industry due to its improved water resistance and excellent bonding to wood. Another advantage of XPVAc to the wood industry is that it emits no formaldehyde during service. In contrast, formaldehyde emission has been plaguing the wood industry, which extensively uses formaldehyde-based synthetic adhesives such as urea formaldehyde (UF), melamine urea formaldehyde (MUF), melamine formaldehyde (MF), PF, resorcinol formaldehyde (RF), phenol resorcinol formaldehyde (PRF) adhesives, and the like. However, XPVAc has never been used as a coating material for wood and wood composites because people always use and treat it as a wood adhesive. It would be desirable to find a means of simply applying a durable and water-resistant coating system on wood and wood composites to ensure a longer service life for them in outdoor applications.